The purpose of a spectrometer is to analyse the spectrum of an input light, by separating the narrow bands of wavelengths composing the input light.
These spectrometers could be used to analyse the spectrum of the light reflected by a sample and to obtain quantitative and/or qualitative information about this sample. By identifying and measuring the narrow bands of wavelength of the light reflected by the sample, it is possible to identify and/or quantify the chemical compounds of the sample.
For that purpose, a spectrometer generally comprises:                an entrance aperture,        an optional optic for transforming the input light into a collimated light (i.e. a light where all the rays composing it are substantially parallel),        a plurality of dispersion or diffraction devices, for producing a spectrum from the input collimated light, and        a plurality of exit apertures.        
A spectrometer further comprises a light detector, for measuring the wavelengths composing the spectrum.
The dispersion device can consist in a prism, and the diffraction device can consist in a diffraction grating, working either in transmission or in reflection.
A light detector measures the intensity of the light projected onto it, and can consist for example in a photodiode.
These spectrometers can be divided in two categories:                the spectrometers measuring simultaneously each narrow band of wavelength of the spectrum, and        the spectrometers measuring sequentially each narrow band of wavelength of the spectrum.        
To measure simultaneously each narrow band of wavelength, referring to the first category of spectrometer, the spectrum shall be projected onto an array of light detectors. Depending on the considered wavelength range, light detectors can be costly and power-consuming. Additionally, a trade-off between the size and the resolution of the array has to be made. Increasing the number of light detectors improves the resolution but also increases the size of the array of light detectors. The resolution is linked to the number of different narrow bands of wavelength measured, and consequently to the width of the narrowest band of wavelength that can be measured. The resolution is improved when the number of measured bands increases or when the width of the bands decreases.
Additionally, the power consumption of the array of light detectors can be high. It could be a concern when designing a handheld spectrometer.
The second category of spectrometer avoids using an array of light detectors by projecting sequentially each narrow band of wavelength onto a single light detector. It is therefore possible to measure successively the whole spectrum by using only one light detector, and consequently resulting in a compacter, less expensive and less power-consuming spectrometer.
Different solutions can be used in order to selectively project a narrow band of wavelength onto a single light detector. For example, when a diffraction grating or a prism is used to generate the spectrum, it is possible to rotate the grating or the prism. This rotation causes the direction of the spectrum to change. By adequately placing the single light detector, it is possible to project each narrow band of wavelength onto the light detector. However, this solution is difficult to implement since it requires mechanical means in order to rotate the grating. Moreover, in this configuration, it is not possible to diffract or disperse the input light more than once with a single grating.
A second solution, disclosed in document WO 2007/089770, is to project the spectrum onto a microelectromechanical system (MEMS), this MEMS being used to selectively reflect a narrow band or a band of wavelength onto a single light detector. However, this solution has the drawback to require one grating and an additional MEMS for the wavelength selection.
Another solution, disclosed in document WO/2007/082952, is to use a tunable grating. The grating is composed of a plurality of beams, each beam having a tilted reflective flat surface. The grating is stretchable, i.e. the distance between the beams is uniformly adjustable. An input collimated light is projected onto the grating, following a direction normal to the flat surfaces. This configuration, where the input light and the diffracted light are in auto-collimation is called the Littrow configuration, and is known to produce the best energetic efficiency of the diffraction. However, it has the drawback to require a circulator or a beam splitter in order to separate the refracted light from the input collimated light, in order to project only the refracted light onto the single light detector. This additional element increases the cost and the size of the system, and can also reduce the energy efficiency (i.e. reduces the intensity of the narrow bands of wavelength). Moreover, this configuration prevents to diffract the input collimated light more than once on the same active area of the diffraction grating.
The present invention proposes a spectrometer in which these drawbacks are avoided.